Seeds several years past their date often still carry a live embryo behind a shell that has hardened too much for the taproot to push through. Scarification and soaking approaches for stubborn stock are explained at https://potentthcseeds.com, including the hydrogen peroxide method. Light sanding on the edge thins the shell without reaching the embryo. The float test people rely on is unreliable, since fresh seeds sometimes sink slowly and dead ones occasionally sink fast.